Viscous liquids are applied by dispensers onto a surface of a substrate in a variety of dispensing applications employed in the manufacture of products and product packaging. These viscous liquids include thermoplastic materials such as hot melt adhesives. Liquid dispensers utilize pneumatically or electrically actuated valve assemblies for metering a precise quantity of the viscous liquid and discharging the metered amount through a small-diameter dispensing orifice. Many thermoplastic materials exist in a solid form at room or ambient temperature and must be heated to create a flowable viscous liquid. Typically, the solid form of material is placed in a holding tank having heated walls and is melted by heating the solid material above its melting point. The viscous liquid is pumped in a molten state under pressure from the holding tank through a supply conduit to a manifold block. The manifold block has liquid passageways connected in fluid communication with the dispensing orifice of one or more liquid dispensers.
Under certain operating conditions, particles can accumulate in the viscous liquid and cause the liquid dispenser to periodically become clogged or fail. For example, to meet the demand of certain high-capacity dispensing operations, the holding tank must store a substantial volume of the viscous liquid and maintain the stored volume within a desired range of operating temperatures. This prolonged exposure to heat in an oxygen-containing environment may cause at least a portion of the viscous liquid to char, oxidize, or otherwise degrade. These conditions produce solidified, insoluble particles that admix with and contaminate the liquid.
The viscous liquid may also be contaminated by particles generated by the manufacturing operations associated with the liquid dispensing operation. If the holding tank is open to the surrounding environment or if a sealed holding tank is opened to add solid thermoplastic material, airborne particles or fibers can fall into the exposed liquid in the tank. Foreign debris that does not dissolve in the liquid will be pumped to the liquid dispenser.
Continuous operation of the liquid dispenser is contingent upon maintaining an unobstructed fluid pathway for the viscous liquid. Particulate contamination in the supply of viscous liquid can disrupt the operation of the liquid dispenser. The dispensing orifice of the liquid dispenser usually defines the maximum tolerable particle size. Particles that lodge in the dispensing orifice can either totally or partially occlude the flow of viscous liquid to the substrate. Particles may also become trapped between the valve and valve seat of the valve assembly and disrupt the metering of precise amounts of viscous liquid. To ensure that the flow of viscous liquid to the liquid dispenser is uninterrupted and that the desired amount of liquid is administered to the substrate, the liquid passageway of the manifold block typically incorporates a filter element. Viscous liquid flowing through the liquid passageway is directed through the filter element, which has a porosity calculated to remove at least the particles of the maximum tolerable size or larger.
When the filter element becomes clogged, the dispensing operation is interrupted to remove the filter element from the manifold block for cleaning or replacement. The downtime of the adhesive dispensing operation associated with the removal, replacement, repair, cleaning, and reinstallation of the filter element results in lost productivity. If the filter element is difficult to remove or reinstall, the servicing downtime further reduces productivity. Both result in additional downtime and lost productivity. Perhaps the most significant impact of a time delay in servicing the filter element is that the viscous liquid in the holding tank remains static and exposed to an oxidizing atmosphere. This further increases particulate contaminants generated by the degrading liquid.
If a filter element is difficult to remove from the manifold block, maintenance personnel may be reluctant to replace or clean the filter element. If the filter element is not regularly serviced in anticipation of the progressive clogging, the system operating pressure will gradually increase. Eventually, the increase in operating pressure may damage either the liquid dispenser or other sensitive components of the dispensing system.
Thus, an improved filter assembly for a liquid dispenser is needed having a configuration that readily permits rapid removal and reinstallation of the filter element from the manifold block.